This invention relates to tanks for the storage of liquids, and, more particularly, to double-wall tanks for underground storage of liquids.
There are currently numerous ways and a variety of materials from which tanks for the storage of liquids are constructed. In one common application, the underground storage of hydrocarbons, such as gasoline and other petroleum products, the tanks have conventionally been fabricated out of steel or fiberglass. Because the earlier tanks were generally fabricated out of a single rigid wall, the increasing age of many of these tanks is beginning to present serious environmental dangers. In addition, many of the older steel tanks buried underground have rusted and are beginning to leak, releasing the petroleum materials into the ground where they may seep into and pollute underground water supplies. Even fiberglass tanks, being generally rust proof, have also exhibited leakage causing similar problems.
One of the main problems with leaking storage tanks has been the difficulty or inability to ascertain when or if such leaks are occurring from a given tank. Because the excavation and removal of such a storage tank, which may contain thousands of gallons of fuel, is an expensive and difficult undertaking, such an operation is difficult to justify unless there is some evidence of actual leakage. Because of the increasing potential danger of leaking storage tanks, particularly in those communities utilizing underground water for public consumption, many municipalities have implemented or plan to implement ordinances requiring the use of double-wall storage tanks and requiring replacement of existing single-wall tanks. While the installation of the conventional double-wall storage tank in a new facility entails no great difficulty and a generally manageable increase in cost over a single-wall tank, the burden of complying with such ordinances by replacing existing sound, single-wall tanks with double-wall tanks can be heavy. This burden has prompted the search for methods of fabricating relatively inexpensive double-wall tanks including remanufacturing existing single wall tanks into a double wall assemblies. One common approach to manufacturing such a double wall tank is preparing a previously used single wall tank by resurfacing and coating it with an appropriate corrosion resistant material, such as the method disclosed in U.S. Pat. No. 4,817,817 to David T. Palazzo, the inventor of the present invention, which is specifically incorporated by reference as though fully set forth herein.
This burden has also given impetus to the search for a method of manufacturing a double-wall assembly having means for detecting the presence of any leaks into the space between two walls. Within the last decade, this search has resulted in several double-wall storage tanks and methods for making them including suitable means for detecting the presence of fluids between the walls of a double-wall tank. Suitable illustration of such double wall tanks having appropriate leak detection devices may be found in the following U.S. Pat. Nos. 4,640,439; 4,644,627; 4,655,367; 4,744,137; and 4,780,947, all of which are invented by David T. Palazzo, the inventor of the present invention, and are incorporated by reference as though fully set forth herein. These patents generally are directed to double wall tanks having a spacing material interposed between the exterior surface of an inner tank and a substantially rigid outer sheath, which tank may further incorporate appropriate devices to detect leaks as well as introduce and withdraw liquids from the tank. In such double wall tank constructions, it is desired to provide a double wall tank that permits substantially free passage of liquids between the outer sheath and the inner tank.
In particular, during the conventional manufacture of such double wall tanks, the outer sheath often bonds to the end portions of the inner tank inhibiting the desired free flow of liquids along the exterior surface of the inner tank. To address this problem U.S. Pat. No. 4,655,367 discloses a method for manufacturing a double-wall storage tank utilizing what is commonly referred to as a formed end cap construction, illustrating the importance of providing free flow of liquids along the end portions of a double-wall tank. This end cap construction, while addressing many of the concerns associated with constructing a double wall tank having the necessary fluid flow along its end portions, requires the additional manufacturing of the end caps as well as added costs.